[28] The purpose of a
wind tunnel is to simulate flight, but in the conventional tunnel the
models of fullscale aircraft are attached to supports so that
aerodynamic forces can be measured. The models are not free. A
plane's maneuverability cannot be gauged completely with these
restrictions on motion.

Since full-scale aircraft are expensive to
build, test fly, and modify-to say nothing of the possible loss of
test pilots-Langley engineers decided to build a free-flight wind
tunnel. The basic idea was to let a model glide in a wind tunnel that
is actually tilted at the aircraft's glide angle. Thus the unpowered
model, nose tilted down at its glide angle, remains stationary and
horizontal in the rising airstream of the tilted tunnel, much like a
hawk or buzzard hovers in air currents. Maneuverability and flight
performance are tested as a "pilot" outside the wind tunnel
manipulates the model's control surfaces via electrical signals sent
through thin wires trailing behind the model.

Two of these tillable free-flight wind tunnels
were constructed at Langley: the first, 5 feet in diameter in 1937
and the second, 12 feet across, in 1939. These tunnels were useful
with radically new aircraft where no reservoir of flight experience
was available, namely, tailless aircraft, planes with delta and
skewed wings, and vertical takeoff and landing/short takeoff and
landing (VTOL/STOL) vehicles. The 12-foot freeflight tunnel was used
into the early 1950s, when it was supplanted by powered models flown
in the Langley full-scale tunnel, which had ample flying room in its
30 x 60-foot test section.